Presently, grinding operations are controlled primarily by establishing timing intervals for each application of an abrasive to an object to be ground and by human observation of the rate at which the abrasives wear away the object being ground. In another control procedure, a grinding operator attempts to set mechanical stops at a level corresponding with the desired grinding depth for each grinding operation.
One such object to be ground is a coupon for a printed circuit board. Printed circuit boards provide the mounting surface and electrical interconnection system for components such as diodes, resistors and capacitors. While originally the circuit boards were only printed on a single side, most circuit boards today are double-sided or multilayered. Multilayer printed circuit boards, that is, circuit boards having circuits that are printed on several layers of the boards, require uniform, dependable electrical connections between the layers. These critical connections are typically provided by plated-through holes prepared by drilling holes through the stacked board layers. The drilled holes are plated with copper and then solder to establish the electrical interconnections. Each and every plated-through hole must maintain a predetermined, uniform thickness: pores, cracks, nodules and other faults in plating render the entire circuit board defective.
Moreover, the reliability of the entire printed circuit board depends on proper adhesion of the solder layer to the copper layer. Solder separation is a serious problem in some situations, particularly for prolonged use at temperatures of 120.degree.-130.degree. F. as discussed in Kumar, K. and Moscaritolo, A., "Optical and Auger Microanalyses of Solder Adhesion Failures in Printed Circuit Boards," 128 J. Electrochem. Soc. 379-383 (1981). Oxide particles accumulate at the interface of the solder layer and the copper layer and eventually cause solder adhesion failure.
A number of printed circuit boards are typically defined on a single panel of material. Fortunately for inspection purposes, plating faults typically appear in most of the plated-through holes or the copper-solder interfaces of a printed circuit board if any faults are present at all. Therefore, one or more coupons are defined on the panel for each circuit board to be printed. For examining the copper-solder interface, each coupon has one or more elongated pads which are plated simultaneously with the printed circuit board. The coupons are labelled with a specific identification code for the individual board and are detached for later examination.
It is desirable to grind the coupons at a slight angle to expose more of the interface. The amplification of the exposed interface is particularly important since the layers are so thin: the copper layer and the solder layer are typically two to three mils thick and the interface is even thinner. Presently, one or more coupons are placed on a Teflon cylinder having a six degree bias. A potting mold is mounted over the coupon and the cylinder and is filled with a potting material to form a first potted surface. The mold is removed, and the partially potted coupons are lifted from the Teflon cylinder and turned upside down. A second mold is mounted over the exposed face of the coupons and additional potting material is added so that the coupons are now completely embedded in potting material which serves as a holder for the coupons.
Unfortunately for inspection purposes, grinding is presently a tedious, time-consuming process since great accuracy is demanded. At least three or four steps are involved which are accomplished manually or semi-automatically by human supervision of a grinding machine. Grinding machines accept one or more holders in a disk which is equipped with a number of adjustable mechanical stops including hardened material such as diamond.
The first potted surface of each holder is ground for several minutes against coarse grit rotating at several hundred rpm. The coarse grit is replaced with medium grit, which is rotated against the holders for an additional period of time. When a semi-automated machine is used, the operator removes the coupons from the medium grit after all diamond stops contact the grit; the operator then resets the diamond stops. Fine grit is then applied against the coupons for thirty to fifty seconds or until all the diamond stops again contact the abrasive. For the grinding machine, the diamond stops are reset flush with the holder. Finally, the coupons with their copper-solder interfaces exposed at a bias are treated in one or more polishing steps.
The sheer cost and labor of grinding one or more coupons per circuit board present serious problems in view of the ever-increasing millions of printed circuit boards that are produced annually. Presently, most coupons are ground manually at the cost of $15-20 per coupon. Manual grinding requires constant operator attention and frequent inspection using a microscope. Each visual inspection interrupts the grinding operation. The dependability of the operator varies greatly: overgrinding and undergrinding occur frequently. Since the coupons are exposed destructively, a mistake in overgrinding is irreparable and results in the complete invalidation of the matching printed wire board unless a second coupon is available for complete regrinding. Undergrinding, when detected, is cured by returning the coupon to the operator, who must remount it and commence additional, unscheduled grinding.
Semi-automated or machine-assisted grinding also requires close operator attention. The machines are more dependable than grinding by hand but are not reliable for exposing the thin interfaces due to cumulative sources of error. The coupons can be misplaced relative to the potting material which results in under- or overgrinding of the coupon. Further, the holder can be initially misaligned in the disk relative to the setting of the mechanical stops, and the stops themselves can wear over time. Also, several holders can be misplaced in a disk relative to each other.
One apparatus attempts to automate grinding by interconnecting two terminal strips with a shorting conductor that is deposited on a substrate to be ground. The grinding operation is controlled by the amount of current flow through the shorting conductor. This technique is relatively inaccurate, however, and relies on final finishing by hand.